The present invention relates to a process for the production of polymer-modified asphalts and asphalt emulsions for use in road construction and repair.
The use of polymer-modified asphalt has received considerable attention in the past few years. Almost everyone in the road building and maintenance industry, including state highway departments, asphalt suppliers, and polymer suppliers, has performed some type of research and development activity or built a trial road with polymer-modified asphalts. The reason for such intense interest in the field is that polymers have been shown to substantially change certain of the physical properties of the asphalt and therefore produce better and longer lasting roads. Some of the properties of asphalt which are affected by the addition of polymers are viscosity, flow, temperature susceptibility, flexibility, and adhesion of aggregates.
Viscosity, flow, and temperature susceptibility are related properties. In general, the addition of polymers to asphalt increases viscosity and flow, and decreases temperature susceptibility. In hot climates, this serves to minimize rutting and deformation of the road surface. Most of these polymers contain butadiene-type monomers imparting "rubber band"-like properties of flexibility and recovery after stretching. This is extremely beneficial in cold climates where cracking presents a problem. In addition, the adhesion of aggregates in chip seals is greatly improved.
Virtually all asphalts used in the United States are products of the distillation of crude petroleum. Asphalt is produced in a variety of types and grades ranging from hard and brittle solids to almost water-thin liquids. Asphalt cement is the basis of all of these products. It can be made fluid for construction uses by heating, by adding a solvent, or by emulsifying it. Hot mix asphalts are used extensively on main highway construction where greater durability is required. When a petroleum solvent, such as naphtha or kerosene, is added to the base asphalt to make it fluid, the product is called a cutback. When asphalt is broken into minute particles and dispersed in water with an emulsifier, it becomes an asphalt emulsion. The tiny droplets of asphalt remain uniformly suspended until the emulsion is used for its intended purpose.
When combined with an appropriate hydrocarbon solvent, the asphalt cement in a cutback is in solution. In an emulsion, the chemical emulsifier is oriented in and around droplets of asphalt cement, thus influencing their dispersion and stable suspension in water. When either a cutback or an emulsion is used in the field, evaporation of the asphalt carrier (i.e., the cutback hydroccarbon solvent or the emulsion water) causes the cutback or emulsion to revert to asphalt cement. In the case of the emulsion, the chemical emulsifier is retained with the deposited asphalt. Because environmental considerations militate against the use of cutback asphalts, due to the necessary solvent expulsion from these applied asphaltic compositions, asphalt emulsions are greatly preferred. These may be divided into three general categories: cationic, anionic, and nonionic emulsions.
Cationic and anionic emulsions are those more commonly used in roadway construction and maintenance. As their names imply, such emulsions utilize anionic or cationic emulsifiers to form an oil-in-water emulsion which can be used alone or combined with aggregate for use in the road construction and maintenance industry.
Emulsions are further classified on the basis of how quickly the asphalt will coalesce, i.e., revert to asphalt cement. The terms RS, MS, and SS have been adopted to simplyfy and standardize this classification. They are relative terms only and mean rapid-setting, medium-setting, and slow-setting, in reference to anionic emulsions. Corresponding rapid-, medium, and slow-setting cationic emulsions are termed CRS, CMS and CSS, respectively. The tendency to coalesce is closely related to the mixing of an emulsion. An RS/CRS emulsion has little or no ability to mix with an aggregate, an MS/CMS emulsion is expected to mix with coarse but not fine aggregate, and an SS/CSS emulsion is designed to mix with fine aggregate.
Chemically, asphalts are complex aggregations of rather large aliphatic and cyclic hydrocarbon molecules. Besides the obvious hydrocarbon content, additional constituents in asphalts may include oxygen, sulfur, and nitrogen (often in substantial quantities) and iron, nickel, and vanadium (present usually in trace quantities). Asphaltic mixtures composed of mineral aggregate and bituminous constituents are used widely in the road construction industry.
Aggregate used in road construction can be hydrophilic or hydrophobic depending upon the nature of the material. While the aggregate can include various mineral materials such as cinders or slags, typically the aggregate is of natural origin such as sand, rock, or the like, typically to the localities where the roads are being built. For example, limestone, dolomite, silica, sedimentary, metamorphic, or igneous rocks of various other kinds are regularly used in road building.
There are three general types of polymers that are currently being used in the asphalt and road building industries, viz., latex polymers, solid polymers, and ground-up automobile tire rubber. The most commonly used latex polymers are neoprene, SBR (styrene-butadiene-rubber), and natural rubber. The most commonly used solid polymers are SBR, EVA (ethylene-vinyl acetate), SBS (syrene-butadiene-styrene), and SIS (styrene-isoprene-styrene).
One of two methods is commonly employed to incorporate polymers into asphalt. One method involves adding latex polymer to an asphalt emulsion either by addition to the emulsifier solution prior to emulsification or to the emulsion following emulsification. Either way, this method is relatively easy and trouble-free. The second method involves adding solid polymer to the asphalt. This method normally requires substantial mixing and shearing in order to uniformly disperse the polymers, particularly when SBS or SIS block copolymers are used.
Two other methods are utilized less frequently to incorporate polymers into asphalt.
Two other methods are utilized less frequently to incorporate polymers into asphalt. One method involves addition of a latex polymer to hot asphalt, whereby the latex is slowly added and the water flashed off. The other method involves addition of solid polymers to asphalt with heating, stirring, and addition of monomers such as styrene or methyl methacrylate. This mixture reacts to yield a chemically crosslinked polymer which is also chemically attached to the asphalt molecules. Thereafter the asphalt is either emulsified or used "as is" in hot applications.
The addition of polymers to asphalt using latex addition either to the emulsifier solution or post emulsification has been used successfully for many years. Initial problems with latex "creaming," floating to the top of the emulsion, and a severe loss in emulsion viscosity have essentially been solved.
The addition of solid polymers to asphalt has not been as successful as latex addition to emulsions. The major reason for this lack of success has been the extreme difficulty encountered in uniformly dispersing the neat polymers in asphalt. Furher, the emulsions produced from the modified asphalt very often possess lower viscosities.
Styrene-butadiene-styrene, styrene-ethylene/butylene-styrene, and styrene-isoprene-styrene block copolymers are being investigated by many asphalt emulsion manufacturers because of the desirable physical properties they impart to asphalt. The very high tensile strength of these block copolymers is caused by the physical crosslinking that occurs when the blocks of styrene orient themselves in rigid domains forming network similar to chemical crosslinking. The rubber mid-block (butadiene, ethylene/butylene, isoprene) gives the polymers their elasticity. Heat, shear and/or solvent will soften the styrene domains and allow flow which facilitates dispersion in asphalt. However, uniform dispersion of the polymers in asphalt requires high-speed, high-shear mixers.
In an attempt to overcome the difficulties of dispersion, manufacturers of the SBS and SIS block copolymers have begun to plasticize various grades of the polymers with aliphatic, aromatic, or naphthenic oils. These oils are intended to separate or pre-dissolve or pre-disperse the styrene domains in order that the resulting mixture of polymer and oil might disperse readily in the asphalt. However, it has been the experience of those skilled in the art that vigorous stirring is nevertheless nearly always required in order to achieve uniform dispersion. Oil-extended blends are now being used in an attempt to alleviate the necessity for such stirring, but have in general resulted in a deterioration of emulsion quality: storage stability is often unacceptable, and emulsion viscosity can be extremely low (i.e., less than 100 SSF at residues as high as 71+%).
Accordingly, there exists a need for a polymer-modified asphalt which can be conveniently and economically prepared without high-speed, high-shear mixing equipment, and for asphalt emulsions prepared from the same polymer-modified asphalt which possess both excellent viscosity and storage stability.